Furnaces for air handling systems are known. Some furnaces are power vented using tubular heat exchangers. Other types of heat exchangers, such as drum/tube and clamshell heat exchangers are also used in some furnaces, but they are in some cases impractical for use in some air handling system configurations for a variety of reasons. In operation, the air to be heated is passed over the outside of the heat exchanger tubes, wherein each tube of the heat exchanger has a burner associated with it. The burners are arranged in a row (either horizontally or vertically) so that a flame on one burner will travel to the remaining burners. An example burner is an ‘inshot’ type burner manufactured by Beckett Gas (see U.S. Pat. No. 5,186,620), and is designed with flame passageways to assist in the flame travel between burners. The burner on one end of the burner row is ignited using an ignition source, for example a sparking or hot surface ignition source, and the flame travels to the remaining burners. A flame sensor at the other end of the burner row verifies that the flame is established along the entire row. A combustion fan draws the air for combustion through the heat exchanger and discharges it outside of the unit.
A flammable gas (typically natural gas or LP gas) is supplied to each burner by a manifold with an orifice feeding gas to each burner. The gas is supplied to the manifold by gas control valve(s) which are electronically controlled. One common configuration is a modulating control with a 4:1 turndown. The turndown is defined as the ratio of the maximum firing rate to the minimum firing rate of the burner and/or furnace. Higher turndown is desirable to achieve better temperature control on mild days. The modulation is achieved using a modulating valve which controls the gas flow to the burners in a variable manner. A shutoff valve (labeled combo valve in the drawings above) is used to shut off gas flow to the furnace when heat is not required. The 4:1 furnace uses a two speed combustion fan to maintain a proper fuel to air ratio at lower firing rates. Other common options for gas control are one stage (on/off) and two stage (high/low/off) control.
Many manufacturers are also using this type of furnace and furnace control in the residential HVAC industry. The level of modulation (turndown) varies from one manufacturer to the next. 2:1 modulation has been around for a long time while 4:1 modulation has been common in the industry for about 15 years. In recent years, manufacturers have been starting to achieve 5:1 modulation more readily and a few have managed 6:1 modulation with the inshot burner/tubular heat exchanger design. However, further improvements in attaining even higher levels of modulation are desired.